Risk v Return
"The question I struggle with is; Should I add to my gaming bankroll?"
As I am not entirely clear on the question (did you mean overall portfolio investment division or just the reinvestment of returns from other investments into blackjack?) Either way, it comes down to risk v return...
For me, Blackjack is just another investment. What it comes down to is risk vs return, and how much risk you are willing to accept for the corresponding return. ie...when deciding between stocks and bonds, depending on my long term goals, I may very well decide on a 70/30 split in favor of stocks, which offer a better return but of course, more risk to bear in return for that. I look at blackjack as a risky investment, probably about as risky as an equity investment in a small relatively unknown company. However, you may consider yourself to be a better player then I do myself, thus you may place a different risk value on your blackjack investment. (or may have more tracked historical data from which to calculate your actual fluctuation).
Overall, you should take a look at your entire investment (blackjack and otherwise), their corresponding returns and determine if the additional return you can obtain from increasing your blackjack bankroll (ie..placing a higher percentage of total investment capital in blackjack, or perhaps reinvesting some of your non-blackjack returns into your bankroll) is worth the incremental risk. Although there are finance calculations you can use to determine this risk vs return ratio, everybody's risk tollerance is different. So generally what I do is just calculate the greater return I will obtain, and then use my intuition to determine if the added risk is worth it. This "intuition" will incorporate your individual risk tollerance level.
Hope some of this helps..
D